Our patent applications Graebner-Jin 10-109 and Graebner-Jin 11-110, filed simultaneously herewith and hereby incorporated herein, teach methods and apparatus for determining the thermal conductivity of electrically insulating crystalline materials. These methods and apparatus require determinations of the thickness and optical absorptivity of a sample body. To these ends it would be desirable to have apparatus for measuring both the optical intensity I emerging from a major surface of the sample body, in response to optical radiation of intensity I.sub.0 incident on an opposing major surface of the sample body, and the thickness t of the sample body. Advantageously, the apparatus should be capable of determining both the emerging optical intensity I and the thickness t simultaneously.